Process for the production of mouldings from materials based on silicon nitride

ABSTRACT

Process for the production of moulded bodies of materials based on silicon nitride, where the powder mix prepared by way of initial material contains silicon nitride (Si 3  N 4 ), a high proportion, preferably 40-85 percent by weight, of silicon (Si) and a sintering aid. With conventional sintering the above powder mix is advantageously prepared by grinding of an Si-powder dispersed in an Si 3  N 4  powder the grain size of which is considerably lower than that of the Si-powder. The process covers in the main the production of a body moulded from this powder mix by known moulding methods, nitriding of the Si-content to Si 3  N 4  and sintering of the moulded body to a final required density.

The present invention relates to a process for the production ofmouldings from materials based on silicon nitride in accordance with thepreamble of patent claim 1.

As a first step in this process a powder mix is prepared bygrinding/mixing silicon nitride with silicon powder containing a highpercentage of Si, preferably 40-85 percent by weight in relation to theamount of Si₃ N₄ added, and a sintering aid, whereby the silicon nitridepowder serves as a dispersing agent for silicon and enables grindingdown of the mix to a mean particle size of less than 1 μm. Afterpreparation of a moulding by means of known moulding methods (pressing,casting, injection moulding etc.) the silicon content of this body is,by way of the next step in this process, partly or fully nitrided tosilicon nitride, thus bringing about an internal increase in mass and,accordingly, an increase in the so-called green density (i.e. compactdensity prior to sintering) of the body produced from powder. Aftermachining, if applicable, the moulding is subjected to final nitridingand is then sintered, preferably by conventional means (withoutsimultaneous application of mechanical pressure) with the assistance ofthe sintering aid added, until a final required density is achieved.

Materials based on silicon nitride are regarded as belonging to thegroup of high-temperature ceramics. The main characteristics of thesematerials are their elevated high-temperature strength, wear resistanceand corrosion resistance, which ensure that they are suitable forcomponents of internal combustion engines as well as for parts subjectto wear and for machining.

When manufacturing products from these materials use is most often madeof traditional ceramic or powder-metallurgical moulding methods such asslip casting, injection moulding, pressing or a combination of theseprocesses. Temporary binders, injection moulding plastics and/orpressing aids are removed in the normal way by burning off (fuming off).Dense materials can be achieved by applying two types of sinteringmethods, i.e. by sintering (in a conventional way or subject tosimultaneous application of mechanical pressure--pressure-sintering) ofbodies made from Si₃ N₄ -powder with an addition of sintering aid, or bynitriding bodies formed from Si-powder (reaction-sintering). As a rule,sintering is effected in a nitrogen atmosphere (possibly subject tooverpressure) at temperatures above 1500° C., whereas reaction-sinteringis generally effected in several stages close to the melting point forpure silicon.

However, these methods are subject to various disadvantages which hindertheir commercial application. It is for instance possible to achieve anentirely non-porous material by pressure-sintering, but onlyuncomplicated shapes can be produced by this method. With reactionsintering it is possible to produce complex shapes with high dimensionaltolerance owing to the fact that the amount of shrinkage which occurs inthe course of nitriding is low. The nitriding process does howeverresult, on the one hand, in a material with a residual porosity ofnormally 15-20% which entails a relatively low strength level, and, onthe other hand, necessitates a treatment period amounting to one orseveral days. It is assumed that this is due to the low rate at whichnitrogen is diffused within the generated microstructure of the siliconnitride. A fine-grained microstructure with an increased proportion ofgrain boundaries can result in increased nitrogen permeability. In orderto bring about such a microstructure it is desirable to start with afinely ground Si-powder, preferably with a grain size of less than 1 μm.However it is hard to grind down silicon to the said grain size owing tothe difficulty of dispersing the ground powder.

With conventional sintering (often described in the literature aspressureless sintering) a completely dense material can be achieved,with a strength comparable to that of pressure-sintered material.Complex mouldings can, in addition, be achieved by known mouldingmethods. So as to achieve a high compact density by conventionalsintering, it is however essential to use powder material with very finegrains--i.e. the mean grain size must be less than 1 μm. This entailsthat the green density of the bodies formed from powder can rarely bemade to exceed 50%. As a result, the amount of shrinkage associated withsintering is high, as a rule 15-20%, making it difficult to maintain therequired dimensional tolerances.

A combined sintering method has also been proposed and made subject of apatent application (A. Giachello and P. Popper in "Energy and Ceramics",Ed. P. Vincenzini (1980) p.620), according to which a body is mouldedfrom silicon powder to which a sintering aid has been added. Followingthe removal of temporary binder the body is first nitrided to a densitycorresponding to that of a reaction-sintered material. At another stage,it is said, the said body can be resintered with the assistance of theadded sintering aid until the full compact density has been reached. Theshrinkage during this process comes to a halt at 6-7%. From the point ofview of process technology the reaction-sintering method is subject to adisadvantage, i.e. it still entails the time-consuming nitridingprocess.

It is also known from the Swedish application No. 800500-1 how to addsilicon to silicon nitride powder when injection moulding bodies forisostatic pressure-sintering. These bodies are subject to considerablemechanical stresses during manipulation in the preparative stages of theisostatic sintering process. By nitriding the Si-content in the mouldedbody (which should amount to 6-60 percent by weight) after burning offthe plastic injection moulding components the strength and, as a result,the suitability for manipulation of the moulded body is claimed to becapable of being increased to a considerable extent. A further advantageconsists in the fact that the time required for nitriding the Si-contentin the moulded body is considerably shorter than with a body consistingonly of Si-powder. The isostatic pressure-sintering method canaccordingly be used also for producing complex mouldings. However, thismethod presupposes costly investments in equipment and it is necessaryto finish the surfaces of the pressure-sintered bodies to a certainextent, thus restricting its commercial usefulness.

A general comparison shows that application of the conventionalsintering method is advantageous, not least from the commercial point ofview, and that it results in material with satisfactory mechanicalcharacteristics. As already stated in the general section it isnecessary with conventional sintering to make use of fine-grained powdermaterial (grain size less than 1 μm). These fine powders are as a ruledifficult to compact. This results in insufficient grain density,especially when use is made of the moulding methods--slip casting andinjection moulding--suitable for large-scale production of mouldingsfrom these materials.

The process according to the present invention aims at overcoming thedisadvantage entailed by this method, i.e. the high degree of shrinkageduring sintering, and at offering a method for achieving this endwithout introducing other parameters capable of complicating the processtechnology.

With the process in accordance with the present invention the greendensity of the moulded bodies can be increased considerably by using asa starting material a powder mix consisting of Si- and Si₃ N₄ -powderand a sintering aid containing a high proportion of silicon, preferablymore than 40 percent by weight in relation to the amount of Si₃ N₄. Inaccordance with the process proposed a body is moulded from this powdermix and after removal (burning off) of temporary binder added at themoulding stage the silicon content is nitrided fully or partly tosilicon nitride. This causes an internal increase in mass withoutdimensional changes in the body moulded from powder and, accordingly, toan increase in the density of the latter. After machining if any--toadjust dimensions--the body is subjected to final nitriding (if onlypartial nitriding has taken place) as a result of which virtually allthe silicon is converted to silicon nitride. It is shown that by using amix of Si and Si₃ N₄, nitrogen will permeate the nitrided body to aconsiderably increased extent. As a result, the nitriding period can bereduced, on average at least to a power of ten, by comparison with theknown reaction-sintering methods. The final stage in the processproposed consists in sintering the body moulded from powder and nitridedwith the assistance of the added sintering aid until a required finaldensity is achieved.

In the powder mix which is used, the silicon nitride shall besubstantially present by way of α-phase, whereas it is known that in thecourse of sintering but without affecting the sintering process as such,the latter is converted to β-phase. This conversion results in a higherstrength level of the sintered material than if β-phase is present inthe initial powder. The silicon in the powder mix used shall be of agrade which, according to the literature in respect of raw materials forreaction-sintered silicon nitride, can be nitrided subject tocorresponding temperature and gas atmosphere conditions such as aresuitable for the production of the reaction-sintered material. The highproportion of silicon in the powder mix used, which constitutes animportant characteristic of this process, is essential for achieving ahigh green density after the nitriding stage. So as to be able toachieve a significant increase in green density, the proportion of Simust accordingly not be less than 40 percent by weight in relation tothe amount of Si₃ N₄. The upper limit for the amount of Si added to themix will depend on the nitriding time required for converting virtuallyall the silicon to silicon nitride. With a view to maintaining theprocess technological advantage of the process proposed, which consistsin a substantial reduction of the nitriding time--by at least a power often by comparison with the known reaction sintering processes--it isadvisable not to use mixes containing more than 85 percent by weight ofSi.

By way of sintering aid the substances described in the scientificliterature may be used, i.e. preferably metal oxides in the conventionalproportions, i.e. 0.1-20 percent by weight. When producing materialbased on silicon nitride of the type SiAlON, i.e. materials based onphases occurring within the system Si₃ N₄ -SiO₂ -Al₂ O₃ -AlN, thecombined amount of other substances added including sintering aids, forinstance Al₂ O₃ and AlN in the important partial system Al₂ O₃ -AlN-Si₃N₄ (material based on β'-phase) may considerably exceed 20 percent byweight.

For nitriding the body moulded from powder use may be made of a separatenitriding oven, but in view of the reduced nitriding time achieved withthis process, nitriding can also be advantageously effected in the ovenused in the next process phase for sintering. The nitrided body canpreferably be sintered by conventional means but pressure-sintering isalso possible. Conventional sintering must be effected in a nitrogenatmosphere. So as to prevent weight losses such as normally occurespecially at the introductory stage of the sintering process owing tothe fact that the material disintegrates, it should also be advisable toeffect sintering at a nitrogen gas pressure at least equal or higherthan 1 atm (0.1 MPa) or/and to make use of the known powder-bedtechnology, with the sintered body packed in protective powder.

For conventional (pressureless) sintering it is essential that thepowder mix used (Si-Si₃ N₄ -sintering aid) consists of a fine-grainedmaterial as stated above. It is also an object of the present inventionto offer a method for producing a powder mix satisfying the need for apowder material capable of being sintered. According to the methodproposed, a fine-grained powder mix is produced by grinding a siliconpowder together with silicon nitride powder and a sintering aid, wherebythe Si-powder used by way of initial material consists of grains theaverage size of which is larger than that of the grains in the Si₃ N₄-powder. With a preferred embodiment the grain size of the Si₃ N₄-powder added prior to grinding is already at the level suitable forconventional sintering. In the course of grinding the addition of Si₃ N₄-powder acts as a dispersant for the Si-powder as a result of which theSi-powder can be ground down to at least approximately the same meangrain size as the Si₃ N₄ -powder added. Si₃ N₄ constitutes a regularcomponent of the powder mix while at the same time acting as adispersant for Si thus facilitating the grinding down of Si to a lessergrain size than if only pure Si were subjected to the same grindingprocedure.

The choice of grain size with the Si-powder used by way of initialmaterial will depend on the required grinding conditions. Studies inaccordance with the method proposed show that these conditions (methodof grinding, grinding bodies, grinding time etc.) can be selected sothat the Si-powder undergoes both grinding and re-mixing, but as regardsthe Si₃ N₄ -powder and the sintering aid added the grinding procedureentails substantially only re-mixing. The above studies have shown thatwith a specific embodiment using a ball mill, the mean grain size mustas a rule not exceed 50-100 μm. In this case the utilisation ofSi-powder of considerably larger grain size than that of the Si₃ N₄added leads both to a considerable lengthening of the grinding time andan increased risk for quantitative ingrinding by grinding bodies andgrinding vessel walls before the dispersing mechanism starts to operate.When applying an effective grinding method, for instance attritiongrinding with grinding bodies consisting of a specially strong,wear-resistant and hard material (Si₃ N₄, SiC, BN) it is howeverpossible to use an even coarser grade of Si-powder by way of initialmaterial.

The grain size of the Si₃ N₄ added prior to grinding shall in accordancewith the method proposed be lower than that of the Si-powder and should,in accordance with the preferred version, be within the range 0.05-1 μm,which is a grain size suitable for conventional sintering. For efficientdispersion during the entire grinding process with the stated Si-contentrange (40-85 percent by weight), the particle size of the Si-powdershould generally exceed that of the Si₃ N₄ -powder by at least a powerof ten. The grain size of the sintering aid added should as a rule beless than that of the Si₃ N₄ already prior to grinding, since thegenerally low proportion added entails only an insignificant degree ofgrinding down and, as a result, a distribution in the powder mixachieved after grinding, which from the point of view of sintering isuneven.

The proposed process and method of implementing the present inventionmay become clearer from the following practical example which describesthe preparation of a powder mix in accordance with the present inventionwith a view to conventional sintering and the process for sintering abody moulded from the said powder mix. The powder mix described in thisexample is prepared from Si₃ N₄ (HC Stark, grade LC10), Si-powder(purity 97.5%) and a sintering aid consisting of Y₂ O₃ and Al₂ O₃. Thespecific surface (BET) characterising the grain size of a powderamounting to 16.3 m² /g (mean grain size <0.1 μm) as regards Si₃ N₄, and0.5 m² /g as regards Si-powder, which also contained a proportion ofgrains up to 50 μm. 45 percent by weight of Si and Si₃ N₄ -powder (50%Si relative to Si₃ N₄) and 7.5 percent by weight Y₂ O₃ as well as 2.5percent by weight Al₂ O₃ by way of sintering aid were introduced intothe grinding vessel of a swinging ball mill. The grinding vessel and thegrinding balls consisted of agate. The mix was wet-ground in ethanol for15 hours, whereupon it was centrifuged in order to separate it fromethanol and finally dried on a water bath. Following this treatment thespecific surface of the mix was measured as 15.0 m² /g, which means thatin the main only Si-powder was subjected to grinding to a grain size ofthe same order of magnitude as the Si₃ N₄ -powder added.

The powder mix was moulded to a body by isostatic pressing at 280 MPawhich resulted in a green density of 58%. The process proposed was thenapplied in two stages--nitriding and sintering. Nitriding was carriedout in a thermal balance in which the absorption of nitrogen wascontinuously recorded as a function of time. The weight of the test bodywhich indicated the amount of nitrogen absorbed was stabilised afterabout 2 hours at 1350° C., this nitriding time being at least by a powerof ten shorter than with the known reaction-sintering process. TheSi-powder applied contained about 1 percent by weight Fe by way ofimpurity, which according to descriptions in the literature should alsohave facilitated nitriding. The increase in weight obtained amounted to20.4%, which means that virtually all Si was converted to Si₃ N₄. On theother hand, the green density after nitriding increased from 58% to 67%and the test body shrunk by about 1%.

Sintering was effected in a graphite resistance oven with the test bodyembedded in a layer of protective powder containing Si₃ N₄, Al₂ O₃ andAlN. The sintering program consisted in treatment for 2.5 hours at 1780°and an aftersintering stage of 0.5 hours at 1850°. Measurements of thetest body carried out after this treatment showed a final density of98.8% of the expected compact density and a linear shrinkage in theregion of 11.5%.

In a corresponding implementation example it also proved possible toeffect the nitriding process in a sintering oven as a result of whichthe entire sintering procedure could be carried out without having tomanipulate the body moulded from powder between the mouldings and thefinal sintering stages.

I claim:
 1. A process for the production of moulded bodies consisting ofmaterials based on silicon nitride comprising preparing a powder mix ofsilicon powder, silicon nitride powder and a sintering aid, mouldingsaid mix, nitriding the Si-content to Si₃ N₄ and sintering,characterised in that the silicon powder is ground down to a meanparticle size below 1 μm in the presence of silicon nitride powder as adispersing agent and in the presence of sintering aids and, that a bodymoulded of this powder mix hereafter is nitrided during a time periodnot longer than 5 (five) hours whereby the nitriding temperature is keptbelow the melting point of pure silicon.
 2. A process in accordance withclaim 1, characterised in that the grain size of the Si-powder addedprior to grinding exceeds that of the Si₃ N₄ -powder.
 3. A process inaccordance with the claim 1, characterised in that the content ofSi-powder is kept within the range of 40-85 percent by weight relativeto the amount of Si₃ N₄ added.
 4. A process in accordance with claim 1,characterised in that nitriding of the body moulded from powder iseffected in the same oven as the subsequent sintering process.
 5. Theprocess in accordance with claim 2, characterised in that the content ofSi-powder is kept within the range of 40-85 percent by weight relativeto the amount of Si₃ N₄ added.
 6. The process in accordance with claim2, characterised in that nitriding of the body moulded from powder iseffected in the same oven as the subsequent sintering process.
 7. Theprocess in accordance with claim 3, characterised in that nitriding ofthe body moulded from powder is effected in the same oven as thesubsequent sintering process.